Cross-hair cell based floating body device

ABSTRACT

A non-planar transistor having floating body structures and methods for fabricating the same are disclosed. In certain embodiments, the transistor includes a fin having upper and lower doped regions. The upper doped regions may form a source and drain separated by a shallow trench formed in the fin. During formation of the fin, a hollow region may be formed underneath the shallow trench, isolating the source and drain. An oxide may be formed in the hollow region to form a floating body structure, wherein the source and drain are isolated from each other and the substrate formed below the fin. In some embodiments, independently bias gates may be formed adjacent to walls of the fin. In other embodiments, electrically coupled gates may be formed adjacent to the walls of the fin.

BACKGROUND

1. Field of Invention

The invention relates generally to electronic devices, and, morespecifically, to non-planar transistors and techniques for fabricatingthe same.

2. Description of Related Art

Fin field effect transistors (finFETs) are often built around a fin(e.g., a tall, thin semiconductive member) extending generallyperpendicularly from a substrate. Typically, a gate traverses the fin byconformally running up one side of the fin over the top and down theother side of the fin. Generally, a source and a drain are located onopposite sides of the gate in the fin. In operation, a current throughthe fin between the source and drain is controlled by selectivelyenergizing the gate.

High aspect ratio fins typically are desirable but challenging toconstruct. Generally, high aspect ratio finFETS can be integrated into asmall area of the substrate, thereby potentially reducing manufacturingcosts on a per-transistor basis. To increase density of the transistors,the width of each fin, and the gap between each fin, may be reduced. Asthe dimensions of the fin structures and the space between each fin arereduced, construction of gates or other structures of the fins may beincreasingly difficult.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 depicts a substrate having features formed during a manufacturingprocess in accordance with an embodiment of the present invention;

FIG. 2 depicts a mask formed during the manufacturing process inaccordance with an embodiment of the present invention;

FIG. 3 depicts row trenches and fins formed during the manufacturingprocess in accordance with an embodiment of the present invention;

FIG. 4 is a side view of a fin of FIG. 3 in accordance with anembodiment of the present invention;

FIG. 5 depicts formation of an oxide in the hollow regions of the fin inaccordance with an embodiment of the present invention;

FIG. 6 is a side view of formation of isolating regions of the fin inaccordance with an embodiment of the present invention;

FIG. 7 depicts formation of independently biased gates in accordancewith an embodiment of the present invention;

FIG. 8 depicts formation of electrically coupled gates in accordancewith an embodiment of the present invention; and

FIG. 9 is a flowchart of a manufacturing process in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION

Some of the subsequently discussed embodiments may facilitate themanufacture of high aspect ratio structures, such as finFETs. As isdescribed in detail below, during manufacture of the finFET, a hollowregion may be formed during etch of the sidewalls of the fins. An oxideor other suitable material may be formed in the hollow region to isolateportions of the transistor from a substrate. The resulting structuresmay form a floating body cell in the body of the fin. The followingdiscussion describes devices and process flows in accordance withembodiments of the present technique. Prior to addressing theseembodiments from the device and process flow perspective, systems inaccordance with embodiments of the present technique are described.

FIG. 1 depicts a substrate 100 having features formed during amanufacturing process in accordance with an embodiment of the presentinvention. It should be appreciated that the features described below inFIG. 1 may be formed by any suitable processes and techniques to form asubstrate suitable for processing to form the floating body cellsdescribed in further detail below.

With reference to FIG. 1, in one embodiment the manufacturing processmay begin with providing a substrate 100. The substrate 100 may includesemiconductive materials such as single crystalline or poly crystallinesilicon, gallium arsenide, indium phosphide, or other materials withsemiconductor properties. Alternately, or additionally, the substrate100 may include a non-semiconductor surface on which an electronicdevice may be constructed such as a plastic or ceramic work surface, forexample. The substrate 100 may be in the form of a whole wafer, aportion of a diced wafer, or a portion of a diced wafer in a packagedelectronic device, for instance.

Additionally, the substrate 100 may include an upper doped region 102and a lower doped region 104 formed in the substrate 100 by any suitableprocesses. The upper doped region 102 and the lower doped region 104 maybe differently doped. For example, the upper doped region 102 may be ann+ material and the lower doped region 104 may be a p− material(referred to as a “p-well”). The depth of the upper doped region 102 maybe generally uniform over a substantial portion of the substrate 100,such as throughout a substantial portion of an array area of a memorydevice, for example. The upper doped region 102 and lower doped region104 may be formed by implanting or diffusing dopant materials.Alternatively, or additionally, one or both of these layers 102 and/or104 may be doped during growth or deposition of all or part of thesubstrate 100, such as during epitaxial deposition of a semiconductivematerial or during growth of a semiconductive ingot from which wafersmay be cut. As is explained below, the upper doped region 102 may form asource and a drain of a transistor, and the lower doped region 104 mayform a channel of a transistor.

One or more layers 105 may be disposed on a surface of the substrate100. For example, such layers may include a pad oxide, a stop body, asacrificial body, and may include such materials as oxides, nitrides,and/or polysilicon. The layer 105 may be used in or may be a remnant ofprocessing of the substrate 100, such as by atomic layer deposition(ALD), chemical vapor deposition (CVD), planarization, etc.

Deep isolation trenches 106 and shallow trenches 108 may be formed inthe substrate 100. These trenches 106 and 108 may generally extend inthe y-direction, as indicated in FIG. 1. One or more shallow trenches108 may be interposed between pairs of the deep isolation trenches 106.In some embodiments, the shallow trenches 108 may be deeper than theupper doped region 102 to separate subsequently formed sources anddrains. Additionally, the deep isolation trenches 106 may be deeper thanthe shallow trenches 108 to isolate subsequently formed transistors. Thedeep isolation trenches 106 and/or shallow trenches 108 may have agenerally rectangular or trapezoidal cross-section, and, in someembodiments, their cross-section may be generally uniform through somedistance in the x-direction. The deep isolation trenches 106 and shallowtrenches 108 may be partially or entirely filled with one or moredielectric materials, such as high density plasma (HDP) oxide, forinstance, to electrically isolate features. Additionally, the deepisolation trenches 106 and/or shallow trenches 108 may include one ormore liner materials, such as silicon nitride for example, to relievefilm stresses, improve adhesion, and/or function as a barrier material.

Turning now to FIG. 2, the manufacturing process may include a row mask110. The row mask 110 may be formed with photoresist or it may be a hardmask, for example, and it may be patterned with photolithography orother lithographic processes, e.g., nano-imprint lithography orelectron-beam lithography. For example, the mask 110 may be formed bypatterning a body of amorphous carbon that is formed on the substrate100. The mask 110 may define masked regions having a width 112 andexposed regions having a width 114. In some embodiments, the row 110 maybe formed with a sub-photolithographic process, e.g., a sidewall-spacerprocess, a resist-reflow process, or a line-width thinning process. Thewidths 112 or 114 may be generally equal to or less than F, ¾ F, or ½ F.The row mask 110 may define a repeating pattern of lines with a pitch116, or in some embodiments, the pattern may be interrupted by otherstructures. The masked regions of the row mask 110 may be generallystraight, generally parallel to one another, and may generally extend inthe x-direction. In other embodiments, the masked regions of the rowmask 110 may undulate side to side or up and down, or they may besegmented.

Next, as shown in FIG. 3, row trenches 118, formed from the exposedregions between the row masks 110, and fins 120 may be formed inaccordance with an embodiment of the present invention. As describedabove, the row trenches 118 may be masked with photoresist and/or byforming a hard mask on the substrate 100. Various sub-photolithographictechniques may be used to pattern the trenches 118, such as reflowingpatterned photoresist and/or forming sidewall spacers on a hard mask,for example. Once a mask is formed, the 118 may be etched from thesubstrate 100 with, for example, by any suitable poly etch, such as ananisotropic etch.

The row trenches 118 may extend in the x-direction, generallyperpendicular to the deep isolation trenches 106 and shallow trenches108. In the present embodiment, the row trenches 118 intersect aplurality of the deep isolation trenches 106 and shallow trenches 108.The row trenches 118 may be generally parallel to each other and ofgenerally uniform depth and width. In some embodiments, the width 114 ofthe row trenches 118 is approximately F/2, where F is the wavelength oflight used to pattern the row trenches 118. However, in otherembodiments, the width 114 may be less than F/2 or greater than F/2. Therow trenches 118 may have a pitch of approximately 4F, greater than 4F,or less than 4F. In a cross-section normal to the x-direction the rowtrenches 118 may be generally rectangular or trapezoidal. Alternatively,the row trenches 118 may have a cross-section with some other shape. Insome embodiments, the cross-section is generally constant through somedistance in the x-direction. The row trenches 118 may be deeper than theshallow trenches 106. In the present embodiment, the sidewalls of therow trenches 118 form walls 122, which, as is subsequently discussed,may each form a first wall or side of a fin 120 having a fin height 124.

As shown in FIG. 3, during etch of the row trenches 118, the portion ofthe substrate 100 underneath the shallow trenches 108 may also be etchedin the x-, y-, and z-directions, forming hollow regions 126 between theshallow trenches 108 and the substrate 100 in the x-, y-, andz-directions. The hollow regions 126 may be any shape (e.g., irregularshape as shown in FIG. 6) and size and the hollow regions 126 may extendbetween deep isolation trenches (such as in the x direction) and mayextend into the substrate 100 (such as in the z-direction into thep-doped substrate). It should be appreciated that the gate oxide formingthe shallow wall trenches 108 and the deep isolation trenches 106remains resistant to the etch or other formation of the hollow regions.The hollow regions 126 generally isolate a source 127 and drain 129 of atransistor formed by the shallow trenches 108 from the channel formed bythe lower doped portion 104 (e.g., p-well) of the substrate 100.

FIG. 4 is a side view of a fin 120 of FIG. 3 in accordance with anembodiment of the present invention. As shown in FIG. 4, the hollowregions 126 are formed (e.g., etched) underneath the shallow trenches108, isolating the source 127 and drain 129 of a transistor. In someembodiments, etching of the row trenches 118 and the hollow regions 126may extend below the fins 120 and into the substrate 100, as shown byregions 128. The etching of the hollow regions 126 may result in aportion 128 being etched at any depth in the substrate 100. However, tomaintain separate (e.g., isolated) transistors of the fin 120, theportion 128 should not extend below the deep isolation trenches 106.Further, the hollow regions 126 should not extend above the bottom 130of the shallow trenches 108 to maintain a transistor channel.

After formation of the hollow regions 126, the hollow regions 126 may befilled with any suitable material. For example, as shown in FIG. 5, thehollow regions may be filled with a gate oxide 134 to form an isolatingregion 136. The gate oxide 134 may be grown on the substrate 100 and maybe grown or deposited in the row trenches 118. In one embodiment thegate oxide 118 may include a high-density plasma (HDP) oxide layerand/or a thermal oxide. The structure 138 formed by the source 127,drain 129, and the isolating region 136 may be referred to as a floatingbody cell, e.g., the source 127 and drain 129 are “floating” above achannel formed in lower doped region 104 of substrate 100.

FIG. 6 is a side view of the floating body cells 138 on the fin 120 inaccordance with an embodiment of the present invention. After growth ordeposition of the gate oxide 134 and formation of the isolating region136, the source 127 of each transistor may be isolated from the drain129 of each transistor, constructing the floating body cell 134. In someembodiments, the floating body cell 134 may be referred to as beingconstructed on a silicon-on-insulator (SOI). For example, the floatingbody cell 134 is constructed on an insulator (e.g., the gate oxide 134or suitable material) disposed on silicon (e.g., the substrate 100).

As shown in FIG. 7, after deposition of the gate oxide 134 and formationof isolating regions 136, independently biased gates 140 and 142 may beformed adjacent to each side of the fins 120 in accordance with anembodiment of the present invention. The gates 140 and 142 may be formedby blanket depositing a conductive material, such as titanium nitride,doped polysilicon, or other conductive material, and spacer etching thematerial to form the gates 140 and 142. The gates 140 and 142 may bedisposed next to the walls 122 of the fin 120 and extend generallyparallel to the fin 120, in the x-direction. The gates 140 and 142 mayextend along any substantial portion of the fin 120 in the x-direction.

In certain embodiments, the fins 120 may form a portion of rows 144 and146 of floating body cells. Each row 144 and 146 may include a pluralityof generally identical floating body cells disposed at generallyequidistant areas along the x-direction. Of course, in otherembodiments, the floating body cells in rows 144 and 146 may not begenerally identical, e.g., n-type and p-type transistors or differentlysized transistors, and/or the floating body cells may not be regularlyspaced along the rows 144 and 146.

As shown in FIG. 7, the gates 140 and 142 may be independently biased toaffect the floating body cells adjacent to each side of the fin 120. Inanother embodiment, gates may be disposed on either side of the fins 120and connected around the ends of each fin 120. FIG. 8 depicts a partialcross-section illustrating electrically coupled gates 148 and 150extending around the end of the fin 120 forming a single structure inaccordance with another embodiment of the invention. The gates 148 and158 may be disposed adjacent to each wall of the fin 120. In such anembodiment, the gates 148 and 150 on either side of the fin 120 may bedependently biased, e.g., they are electrically connected and biasedtogether. As shown in FIG. 8, the gates 148 and 150 may form acontinuous structure around an end 152 of the fin 120.

FIG. 9 depicts one embodiment of a manufacturing process 200 that may beused to manufacture a finFET or other high aspect ratio structureshaving floating body cells. With reference to FIG. 9, the manufacturingprocess 200 may begin with providing a substrate 100, as depicted byblock 202. The substrate 100 may include any of the materials discussedin reference to the substrate 100 in FIG. 1. Additionally, the substrate100 may include formation of the upper doped region 102 and a lowerdoped region 104, as depicted by block 204 in FIG. 12. It should benoted that the step depicted by block 204, like many of the steps in themanufacturing process 200, may be performed in a different sequence thanthat depicted by FIG. 9.

Deep isolation trenches 106 and shallow trenches 108 may be formed inthe substrate 100, as depicted by block 206 in FIG. 9. The manufacturingprocess 200 may include depositing or growing a fin mask, as depicted byblock 208 in FIG. 9. Next in the manufacturing process 200, row trenches118 may be formed, as depicted by block 210 in FIG. 9, by any suitableprocess, such as anisotropic etch. As described above and shown in block212, during the formation of row trenches 118, the hollow regions 126may be formed underneath the shallow trenches 108 and between the deepisolation trenches 106.

After formation of the hollow regions, the manufacturing process 200 mayinclude growing or depositing the gate oxide 134 in the hollow regions126, as shown in block 214. As described above, the gate oxide 134 mayinclude a high-density plasma (HDP) oxide layer and/or a thermal oxide.In one embodiment of the manufacturing process 200, independently biasedgates 140 and 142 may be formed on the walls of the fins 120, asdepicted by block 212 in FIG. 9. In other embodiments, electricallyconnected gates 148 and 150 (e.g., dependently biased active gates) maybe formed on the walls of the fins 120, as depicted by block 218.

1. A memory device, comprising: a fin; and a floating body cell formedin the fin, wherein the floating body cell comprises a source and adrain separated by a dielectric portion formed in a trench in the fin,and wherein the source and drain are isolated by an isolating regionformed beneath the trench.
 2. The memory device of claim 1, comprising afirst gate disposed on a first wall of the fin.
 3. The memory device ofclaim 1, comprising a second gate disposed on a second wall of the fin,wherein the second gate is configured to be independently biased fromthe first gate.
 4. The memory device of claim 2, comprising a secondgate disposed on a second wall of the fin, wherein the second gate iselectrically coupled to the first gate.
 5. The memory device of claim 1,wherein the isolating region comprises a thermal oxide, a high densityplasma oxide, or a combination thereof.
 6. The memory device of claim 1,wherein the isolating region extends between a first isolation trenchand a second isolation trench of the floating body cell.
 7. The memorydevice of claim 1, wherein the fin has a fin height that is generallyperpendicular to a substrate and a fin width that is generally parallelto the substrate.
 8. The memory device of claim 1, wherein the isolatingregion extends through the fin width and a portion of the fin height. 9.A memory device, comprising: a fin having a plurality of floating bodycells disposed longitudinally along the fin, herein each floating bodycell comprises a source, a drain, a trench, and an isolating regionformed underneath the channel and separating the source and drain from asubstrate; a first gate disposed along a first wall of the fin; and asecond disposed along a second wall of the fin, wherein the first gateis electrically independent of the second gate.
 10. The memory device ofclaim 9, wherein the first gate comprises a conductive material and thesecond gate comprises the conductive material.
 11. The memory device ofclaim 9, wherein the fin comprises a plurality of deep isolationtrenches, wherein each deep isolation trench is disposed between each ofthe plurality of floating body cells.
 12. The device of claim 9, whereinthe isolating region comprises an oxide grown or deposited on thesubstrate.
 13. A memory device, comprising: a fin having a plurality offloating body cells disposed longitudinally along the fin, herein eachfloating body cell comprises a source, a drain, a trench, and anisolating region formed underneath the trench and separating the sourceand drain from a substrate; a first gate disposed along a first wall ofthe fin; and a second disposed along a second wall of the fin, whereinthe first gate is electrically coupled to the second gate.
 14. Thememory device of claim 13, wherein the first gate and the second gatecomprise a continuous structure formed around an end of the fin.